An insight into the removal of Pb(II), Cu(II), Co(II), Cd(II), Zn(II), Ag(I), Hg(I), Cr(VI) by Na(I)-montmorillonite and Ca(II)-montmorillonite

Heavy metal pollution has attracted extensive attention due to the dangers to ecosystems and human health. In this work, Na-montmorillonite and Ca-montmorillonite were used to remove Pb(II), Cu(II), Co(II), Cd(II), Zn(II), Ag(I), Hg(I), and Cr(VI) from aqueous solution. Na-montmorillonite (Na-Mt) and Ca-montmorillonite (Ca-Mt) were characterized using X-ray diffraction, scanning electron microscopy, and nitrogen surface area. The effects of contact time and initial concentration on metal removal were studied. A pseudo-second-order kinetic model and Langmuir, Freundlich, and Dubinin–Radushkevich isotherms were used to describe the adsorption behavior. The heavy metal removal mechanism was discussed in terms of solution pH variation during the adsorption and changes of Na and Ca concentrations before and after adsorption. The results indicated the capacity of Na-Mt and Ca-Mt to Pb(II), Cu(II), Co(II), Cd(II), Zn(II) reached 0.262, 0.152, 0.205, 0.204, 0.151mmol/g and 0.165, 0.104, 0.175, 0.159, 0.116mmol/g, respectively. The calculated adsorption free energy of between 8kJ/mol and 16kJ/mol indicated ion exchange was the main adsorption mechanism. Besides, the role of precipitation was more important for Na-Mt heavy metal ion adsorption than for Ca-Mt. The Na-Mt was more effective for heavy metal adsorption than Ca-Mt and proved to be a potentially useful material for Pb(II), Cu(II), Co(II), Cd(II), Zn(II) removal from aqueous solution.